The present invention relates to a semiconductor light emitting device such a semiconductor laser or a light emitting diode that is capable of emitting a light in a blue region required for higher definition of an optical disc memory having a high recording density, a laser beam printer, etc. which use a ZnO based compound semiconductor, and a method for manufacturing the same, and a device using a ZnO based compound semiconductor such an SAW device, a pyroelectric device, a piezo-electric device, a gas sensor, etc. and a method for growing a crystal of a ZnO based compound semiconductor layer used in the manufacture of the same. More particularly, the present invention relates to a device using a ZnO based compound semiconductor that is capable of providing electrodes on both right and back sides of a chip and also capable of cleavage or that is capable of growing a ZnO based compound semiconductor layer with good crystallinity to improve element characteristic such as light emitting efficiency, and a method for growing a ZnO based compound semiconductor layer for the manufacture of such devices.
Recently, blue region (which means to be of a wavelength range of ultra-violet to yellow colors) light emitting diodes (hereinafter abbreviated as LEDs) for use as a light source used in a full-color display and signal lamp or blue region semiconductor lasers (hereinafter abbreviated as LDs) for a light source used in a next-generation high-definition DVD capable of continuous oscillation at room temperature, were developed by growing GaN-based compound semiconductor layers on the C-plane of a sapphire substrate, and thereby drawing attention from the industries.
As shown in FIG. 14 illustrating a perspective explanation view of an LD chip, this structure comprises a sapphire substrate 21 and group III nitride compound semiconductor layers sequentially grown thereon by Metal Organic Chemical Vapor Deposition (hereinafter abbreviated as MOCVD), in such a configuration that a GaN buffer layer 22, an n-type GaN layer 23, an n-type clad layer 24 made of Al0.12Ga0.88N, an n-type light guide layer 25 made of GaN, an active layer 26 having a multi-quantum well structure made of InGaN-based compound, a p-type light guide layer 27 made of p-type GaN, a first p-type clad layer 28a made of p-type Al0.2Ga0.8N, a second p-type clad layer 28b made of Al0.12Ga0.88N, and a contact layer 29 made of p-type GaN are stacked sequentially and parts of these stacked semiconductor layers are dry-etched, as shown in FIG. 14, to expose the n-type GaN layer 23 on which an n-side electrode 31 is formed with a p-side electrode 30 being formed on the contact layer 29.
The ZnO based compound semiconductor, on the other hand, has been studied in a variety of aspects because it has wide-gap energy semiconductor so that Cd can be mixed in crystallinity to narrow the band gap energy and also emit a blue region light similarly and also because it can be used in a SAW device, a pyro-electric device, a piezoelectric device, etc. This ZnO based compound semiconductor is also a hexagonal crystal like GaN based compound and sapphire, and so has an approximate value of the lattice constant, so that (0001) sapphire having a C-plane as its main plane which is used generally as a substrate for growing epitaxially GaN based compound semiconductor layers in the industries is expected to be used as a substrate for growing ZnO based compound semiconductor layers.
The growth of a ZnO based compound semiconductor on the (0001) sapphire substrate is described in, for example, xe2x80x9cRoom-temperature ultraviolet laser emission from self-assembled ZnO microcrystallite thin filmsxe2x80x9d, Applied Physics Letters, Vo. 72, No. 25, issued on Jun. 22""nd, 1998, pp. 3270-3272.
As mentioned above, since a substrate employed in the prior art blue region semiconductor light emitting device is made of (0001) sapphire having the C-plane as its main plane, it has no conductivity and cannot give therein a vertical-type device (which means to have a construction in which electrodes are provided on both right and back sides of the chip) that has electrodes on both its top and bottom faces of a stack. These electrodes, therefore, must be provided on the upper surface of the semiconductor layer lamination and the surface of its underlying semiconductor layer exposed by etching part of the upper layers, thus giving birth to a problem of complicating manufacturing processes such as etching process and chip die bonding process. Moreover, since the sapphire substrate is very hard, it cannot be cloven easily and is difficult to form a flat end face necessary as the mirror surface of an optical resonator for a semiconductor laser, problematically. That is, although the sapphire substrate is capable of obtaining a well-conditioned mono-crystal semiconductor layer, it has an inevitable difficulty in processibility and formation of electrodes during the manufacturing process.
Further, sapphire has a c-axial length Cs of 1.2991 nm and an a-axial length as of 0.4754 nm, while ZnO has a c-axial length cz of 0.5213 nm and an a-axial length az of 0.325 nm, so that the lattice mismatching rate xcex5 becomes a very large value of xcex5=(azxe2x88x92as)/as=xe2x88x9231.6%. In this case, as shown in FIG. 15, the ZnO crystal may sometimes grow as rotated by 30 degrees, even in which case, the crystal mismatching degree xcex5 has a very large value of xcex5=(⅔xc2xdxc2x7azxe2x88x92zs)/as=xe2x88x9221.1%. This brings about complicated actions of such various parameters as a substrate temperature at the time of crystal growth, amounts of Zn and O elements supplied, a substrate surface treatment method, and an inclination angle, thus giving birth to a problem of poor reproducibility of the flatness of a crystal-growing surface.
Also, since sapphire and ZnO mismatch in lattice constant with each other, the ZnO crystal may sometimes grow as rotated by 30 degrees as mentioned above, so that there are mixed a crystal not rotated and a crystal rotated by 30 degrees, thus giving birth to a problem of even poorer reproducibility of the flatness of the crystal growing surface.
In view of the above, it is a first object of the present invention to provide a semiconductor light emitting device such as an LED or LD made of a ZnO based compound semiconductor, that is of a vertical type capable of providing electrodes from both right and back sides of a chip thereof, that has excellent crystallinity of a semiconductor layer thereof and a good light emitting efficiency, and that does not use sapphire as a material of a substrate thereof to thereby provide a convenient construction in both manufacture and use.
It is a second object of the present invention to provide a semiconductor light emitting device manufacturing method involving surface treatment of a silicon substrate fitted especially to a purpose of growing a ZnO based compound semiconductor on the silicon substrate with good crystallinity.
It is a third object of the present invention to provide a device using a ZnO based compound such as a semiconductor light emitting device with improved device properties which gives a ZnO based compound crystal layer with good crystallinity even on a sapphire substrate.
It is a fourth object of the present invention to provide such a method for growing a ZnO based compound layer that is capable of giving a ZnO based compound crystal layer with excellent crystallinity even on a sapphire substrate.
It is a fifth object of the present invention to provide a semiconductor light emitting device such as an LED or LD having excellent light emitting properties which employs a ZnO based compound semiconductor while using a sapphire substrate.
The present inventors greatly investigated about how to grow a ZnO based compound semiconductor on a large-diameter and easy-to-handle silicon substrate by eliminating inconveniency of growing the ZnO based compound semiconductor on a sapphire substrate as mentioned above, to achieve the first and second objects. As a result, it was found that a reason why an attempt to grow a ZnO based compound semiconductor directly on a silicon substrate results in an amorphous ZnO based compound to thereby disable obtaining a semiconductor layer with good crystallinity is that radical oxygen introduced to grow the ZnO based compound acts to fiercely oxidize the surface of the silicon substrate first to there make the surface amorphous before ZnO based compound grows, so that by forming beforehand a thin nitride film by nitridating the surface of the silicon substrate, the surface of the silicon substrate can be prevented from being oxidized to thereby grow a ZnO based compound semiconductor layer excellent in crystallinity, thus obtaining a semiconductor light emitting device having excellent light emitting properties.
To achieve the first object, a semiconductor light emitting device according to the present invention includes a silicon substrate, a silicon nitride film formed on the surface of the silicon substrate, and a semiconductor layer lamination which is formed on the silicon nitride film and also which has at least n-type and p-type layers made of an ZnO based compound semiconductor to thereby form a light emitting layer.
A ZnO based compound semiconductor referred to here specifically means ZnO or an oxide of Zn and one or more group IIA elements, Zn and one or more group IIB elements, or Zn and one or more group IIA and group IIB elements. This is the same in the following description.
By providing such a construction, in which a silicon nitride film is formed on the surface of a silicon substrate, even when radical oxygen is introduced to grow a ZnO based compound semiconductor layer, the surface of the silicon substrate is not roughened by oxidation, thus enabling growing on the surface a ZnO based compound semiconductor layer with good crystallinity. As a result, it is possible to obtain a semiconductor layer lamination with good crystallinity and hence a semiconductor light emitting device excellent in light emitting properties.
Preferably the surface of the silicon nitride film is not formed amorphous but flat to thereby further improve the crystallinity of a ZnO based compound semiconductor later formed thereon.
In this specification, to be flat in surface of the silicon nitride film refers to such a surface condition that the surface is not amorphous nor irregular so its lattice array may be recognizable, for example, such a condition that a streaky or spotty image may appear as a result of, for example, reflective high-energy electron diffraction (RHEED, by which an electron beam accelerated at 10-50 kV is made incident upon a substrate surface with a small angle (1-2 degrees) to thereby project the electron beam reflected and diffracted by surface atoms onto a screen in order to check the crystal surface condition).
The silicon nitride film should preferably be formed to a thickness of not more than 10 nm so that its surface may not be poly-crystallized but be flat.
Preferably the semiconductor layer lamination has a double-hetero construction that sandwiches an active layer made of CdxZn1xe2x88x92xO (0xe2x89xa6x less than 1) between clad layers which are made of MgyZn1xe2x88x92yO (0xe2x89xa6y less than 1) and also each of which has a larger band gap energy than that of the active layer, to thereby provide an LED or LD made of a ZnO based compound semiconductor and excellent in light emitting properties.
To achieve the second object, a method for manufacturing a semiconductor light emitting device of the present invention includes the steps of; forming a silicon nitride film on a surface of a silicon substrate by conducting heat treatment the silicon substrate in an atmosphere containing nitrogen, and growing on the silicon nitride film a semiconductor layer lamination to form a light emitting layer which is made of a ZnO based compound semiconductor.
This method makes it possible to form a nitride film on the surface of a silicon substrate so that the surface may not be oxidized and, at the same time, not be poly-crystallized to thereby maintain a crystallized surface of the silicon substrate on which a ZnO based compound semiconductor excellent in crystallinity can be grown and also to form the silicon nitride film very thin to thereby prevent electrical discontinuity between the silicon substrate and the semiconductor layer lamination.
To prevent the poly-crystallization, preferably the step of forming the silicon nitride film is performed while controlling the processing temperature or time in such a manner that the surface of the silicon nitride film formed may maintain the flatness of the silicon substrate surface. That is, preferably the nitridation processing is conducted at 650xc2x0 C. for 5 to 10 minutes or so and, more preferably, for 7 minutes or so to obtain a ZnO based compound semiconductor layer excellent in crystallinity; if it is conducted for 15 minutes or so, however, the surface is poly-crystallized to poly-crystallize a ZnO based compound semiconductor grown thereon also, so that it is impossible to obtain a ZnO based compound semiconductor layer with good crystallinity. At a temperature of 800xc2x0 C., on the other hand, nitridation processing needs to be conducted only for 3 minutes or so to obtain a ZnO based compound semiconductor layer excellent in crystallinity; while at the lower temperature, conversely, the processing time should preferably be longer. Those conditions can be set so as to provide a flat surface of the silicon nitride film by checking the surface condition using, for example, the above-mentioned RHEED method.
To achieve the third through fifth objects, the present inventors greatly investigated to grow a ZnO based compound layer with fewer lattice defects and better crystallinity in order to grow a ZnO based compound crystal layer on a sapphire substrate. As a result, it was found that a ZnO based compound layer can be grown on the surface of a sapphire substrate by growing on a main face such an A-face that is perpendicular to the C-face, to thereby obtain a device very excellent in crystallinity as well as in device properties such as light emitting properties.
To achieve the third object, a device having a ZnO based compound layer of the present invention comprises a sapphire substrate which has its main face that is perpendicular to the C-face thereof and a ZnO based compound layer grown epitaxially on the main face of the sapphire substrate.
The term xe2x80x9cface perpendicular to the C-face of a sapphire substratexe2x80x9d here includes, besides the sapphire""s A-face, a face that crosses at right angles with the C-face, which may rotate in the C-face plane and the term xe2x80x9cperpendicularxe2x80x9d implies an allowance of xc2x10.5 degree, a typical value in the substrate specifications.
In such a construction, a ZnO based compound layer grows perpendicularly to the c-axis of the sapphire substrate, so that the a-axis of ZnO grows along the c-axis of sapphire. As a result, presumably, four crystals of the ZnO based compound each measuring an a-axial length (of 0.325 nm) go along a c-axial length (of 1.2991 nm) to thereby give a very low crystal mismatching degree of 0.07% or so, thus providing an excellent crystal face.
Preferably the sapphire substrate has an A-face as its main face because it is easy to get.
To achieve the fourth object, a method for growing a ZnO based compound layer of the present invention features that the ZnO based compound layer is epitaxially grown on a sapphire substrate so that the c-axis of the ZnO based compound layer may be perpendicular to the c-axis of the sapphire substrate.
To achieve the fifth object, a semiconductor light emitting device of the present invention includes a sapphire substrate having its main face that is perpendicular to the C-face thereof, and a semiconductor layer lamination which layers are laminated to form a light emitting layer, the layers having at least n-type and p-type layers made of a ZnO based compound semiconductor grown epitaxially on the main face of the sapphire substrate. The main face of the sapphire substrate perpendicular to the C-face may be, for example, the A-face.
Preferably the semiconductor layer lamination has a double-hetero construction that sandwiches an active layer made of CdxZn1xe2x88x92xO (0xe2x89xa6x less than 1) by clad layers which are made of MgyZn1xe2x88x92yO (0xe2x89xa6y less than 1) and also each of which has a larger band gap energy than that of the active layer to thereby obtain an LED or LD made of an ZnO based compound semiconductor and excellent in light emitting properties.